Boiler Pressue

[Jimbobmattison]

Hi Guys, I hope someone can help me out with this dilemma.

Basically, our boiler needs topping up quite regularly as the pressure is down to 0, mostly in the mornings.

It’s important to mention however that this problem is quite intermittent, we can go weeks without a problem followed by several days constantly having to top it up. I don't know what the problem is and I've looked for leaks and can’t find any + the pipe outside is not leaking.

If it helps, we notice it drops when the heating is on FULL or on very low but again it is intermittent, sometimes months can go by without having to top it up at all.

edit: the pressure we put it to is usually around 1.5 which goes up to around 3 bars when the heating is on. Then if it having a bad day, goes to 0 pressure!!
Please help...

 

[Boilerman2]

Expansion vessel sounds to be suspect!
with the system pressure a Zero and open check the expansion vessel with a tyre pressure gauge, it may need pumping up to 1-00Bar
you can then refill the water to 1.2Bar and all should then be well

(See Sticky on re-charging an Expansion vessel)

 

[Jimbobmattison]

HI Boilerman,

to be fair, a plumber did look at this last year and topped it up and it was ok for 6 months. But then it started happening again and during a recent visit from British Gas they said there was air in the expansion valve so that wasn't the issue. But then again they are useless.

If it is the expansion valve, why would it lose its pressure?

Many Thanks for your reply.

 

[MANDATE]

Expansion vessels have a 'water' side and an 'air' side. The'air' side is pumped up like a car tyre using a air pump, so maybe the shrader valve is not a perfect seal. You wouldn't notice it and gradually as the heating is used and the water expands it compresses the air (helps shrader valve to leak at faster rate).
The overall effect is that the volume of air in the vessel reduces. Now if the volume is halved, then pressure is basically doubled.
So a normal operational pressure rise rise of say 1.5bar ( 1bar to 2.5bar) would become 3bar ( 1bar to 4bar)
The pressure relief valve opens at 3bar so it never reaches the 4bar, but you have lost basically all the pressure and topping up is required.
Of course if the problem has not been corrected, the situation will just repeat itself.
The most important aspect regarding the expansion vessel is making sure the vessel contains max air and no water.
As boilerman2 puts it 'system pressure at zero and open' meaning open to atmosphere.
PRVs once opened can also be the source of a small leak. Dirt on the valve seat may allow water pressure to drop overnight without the heating being on and it may not even be seen at the PRV outlet pipe due to evaporation.